Transversely-excited film bulk acoustic resonator with diaphragm support pedestals

ABSTRACT

Acoustic resonator devices and methods are disclosed. An acoustic resonator device includes a substrate having a surface and a piezoelectric plate having front and back surfaces. The back surface of the piezoelectric plate is attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. One or more diaphragm support pedestals extend between the substrate and the diaphragm within the cavity.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

RELATED APPLICATION INFORMATION

This patent is a continuation of application Ser. No. 17/030,063, filedSep. 23, 2020 entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATORS WITH DIAPHRAGM SUPPORT PEDESTALS which claims priority fromprovisional patent application 62/904,143, filed Sep. 23, 2019, entitledXBAR RESONATOR WITH SI SUPPORT UNDER IDT METAL, and which is acontinuation-in-part of application Ser. No. 16/829,617, entitled HIGHPOWER TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS ON Z-CUTLITHIUM NIOBATE, filed Mar. 25, 2020, which is a continuation ofapplication Ser. No. 16/578,811, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATORS FOR HIGH POWER APPLICATIONS, filed Sep. 23, 2019,now U.S. Pat. No. 10,637,438. which is a continuation-in-part ofapplication Ser. No. 16/230,443, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, filed Dec. 21, 2018, now U.S. Pat. No. 10,491,192,which claims priority from the following provisional patentapplications: application 62/685,825, filed Jun. 15, 2018, entitledSHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20, 2018,entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filed Oct. 5,2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR);application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE FILMBULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR. All of these applications are incorporated herein byreference.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. These bands are not presently used. Future proposalsfor wireless communications include millimeter wave communication bandswith frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view and two schematic cross-sectional viewsof a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3 is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 4 is a graphic illustrating a shear primary acoustic mode in anXBAR.

FIG. 5 is a schematic block diagram of a filter using XBARs.

FIG. 6 is a schematic cross-sectional view of two XBARs illustrating afrequency-setting dielectric layer.

FIG. 7A is a schematic plan view of an XBAR with diaphragm supportpedestals.

FIG. 7B is a schematic cross-sectional view of the XBAR with diaphragmsupport pedestals of FIG. 7A.

FIG. 8A is a detailed schematic cross-sectional view of an XBAR withdiaphragm support pedestals.

FIG. 8B is an alternative detailed schematic cross-sectional view of anXBAR with diaphragm support pedestals.

FIG. 9A is a schematic plan view of another XBAR with diaphragm supportpedestals.

FIG. 9B is a schematic cross-sectional view of the XBAR with diaphragmsupport pedestals of FIG. 9A.

FIG. 10 is a schematic plan view of another XBAR with diaphragm supportpedestals.

FIG. 11 is a is a flow chart of a process for fabricating an acousticresonator or filter.

FIG. 12 is a is a flow chart of another process for fabricating anacoustic resonator or filter.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are well suited for use infilters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having a front surface 112 and aback surface 114. The front and back surfaces are essentially parallel.“Essentially parallel” means parallel to the extent possible withinnormal manufacturing tolerances. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the front surface 112 and back surface114. However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations including rotated Z-cut and rotatedYX-cut.

The back surface 114 of the piezoelectric plate 110 is attached to asurface 122 of the substrate 120 except for a portion of thepiezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140formed in the substrate 120. The portion of the piezoelectric plate thatspans the cavity is referred to herein as the “diaphragm” due to itsphysical resemblance to the diaphragm of a microphone. As shown in FIG.1, the diaphragm 115 is contiguous with the rest of the piezoelectricplate 110 around all of a perimeter 145 of the cavity 140. In thiscontext, “contiguous” means “continuously connected without anyintervening item”.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be attached to the substrate 120using a wafer bonding process. Alternatively, the piezoelectric plate110 may be grown on the substrate 120 or otherwise attached to thesubstrate. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers.

The cavity 140 is an empty space within a solid body of the resonator100. The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 (as shown subsequently in FIG. 3). The cavity 140 may be formed, forexample, by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. An IDT is an electrode structure for convertingbetween electrical and acoustic energy in piezoelectric devices. The IDT130 includes a first plurality of parallel elongated conductors,commonly called “fingers”, such as finger 136, extending from a firstbusbar 132. The IDT 130 includes a second plurality of fingers extendingfrom a second busbar 134. The first and second pluralities of parallelfingers are interleaved. The interleaved fingers overlap for a distanceAP, commonly referred to as the “aperture” of the IDT. Thecenter-to-center distance L between the outermost fingers of the IDT 130is the “length” of the IDT.

The term “busbar” refers to the conductors that interconnect the firstand second sets of fingers in an IDT. As shown in FIG. 1, each busbar132, 134 is an elongated rectangular conductor with a long axisorthogonal to the interleaved fingers and having a length approximatelyequal to the length L of the IDT. The busbars of an IDT need not berectangular or orthogonal to the interleaved fingers and may havelengths longer than the length of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate that spans, or is suspended over, the cavity140. As shown in FIG. 1, the cavity 140 has a rectangular shape with anextent greater than the aperture AP and length L of the IDT 130. Acavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may more or fewer than foursides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device willhave more than ten parallel fingers in the IDT 110. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 110.Similarly, the thickness of the fingers in the cross-sectional views isgreatly exaggerated in the drawings.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g.bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000nm.

A front-side dielectric layer 214 may be formed on the front side of thepiezoelectric plate 110. The “front side” of the XBAR is the surfacefacing away from the substrate. The front-side dielectric layer 214 hasa thickness tfd. The front-side dielectric layer 214 is formed betweenthe IDT fingers 238. Although not shown in FIG. 2, the front sidedielectric layer 214 may also be deposited over the IDT fingers 238. Aback-side dielectric layer 216 may be formed on the back side of thepiezoelectric plate 110. The back-side dielectric layer 216 has athickness tbd. The front-side and back-side dielectric layers 214, 216may be a non-piezoelectric dielectric material, such as silicon dioxideor silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfdand tbd are typically less than the thickness ts of the piezoelectricplate. tfd and tbd are not necessarily equal, and the front-side andback-side dielectric layers 214, 216 are not necessarily the samematerial. Either or both of the front-side and back-side dielectriclayers 214, 216 may be formed of multiple layers of two or morematerials.

The IDT fingers 238 may be one or more layers of aluminum, asubstantially aluminum alloy, copper, a substantially copper alloy,beryllium, gold, molybdenum, or some other conductive material. Thin(relative to the total thickness of the conductors) layers of othermetals, such as chromium or titanium, may be formed under and/or overthe fingers to improve adhesion between the fingers and thepiezoelectric plate 110 and/or to passivate or encapsulate the fingers.The busbars (132, 134 in FIG. 1) of the IDT may be made of the same ordifferent materials as the fingers. As shown in FIG. 2, the IDT fingers238 have rectangular cross-sections. The IDT fingers may have some othercross-sectional shape, such as trapezoidal.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e., the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness ts of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3 is an alternative cross-sectional view along the section planeA-A defined in FIG. 1. In FIG. 3, a piezoelectric plate 310 is attachedto a substrate 320. A portion of the piezoelectric plate 310 forms adiaphragm 315 spanning a cavity 340 in the substrate. The cavity 340does not fully penetrate the substrate 320. Fingers of an IDT aredisposed on the diaphragm 315. The cavity 340 may be formed, forexample, by etching the substrate 320 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings (not shown) provided in the piezoelectricplate 310. In this case, the diaphragm 315 may be contiguous with therest of the piezoelectric plate 310 around a large portion of aperimeter 345 of the cavity 340. For example, the diaphragm 315 may becontiguous with the rest of the piezoelectric plate 310 around at least50% of the perimeter 345 of the cavity 340.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. A radio frequency (RF) voltage is applied to the interleavedfingers 430. This voltage creates a time-varying electric field betweenthe fingers. The direction of the electric field is primarily lateral,or parallel to the surface of the piezoelectric plate 410, as indicatedby the arrows labeled “electric field”. Since the dielectric constant ofthe piezoelectric plate is significantly higher than the surroundingair, the electric field is highly concentrated in the plate relative tothe air. The lateral electric field introduces shear deformation, andthus strongly excites a shear-mode acoustic mode, in the piezoelectricplate 410. Shear deformation is deformation in which parallel planes ina material remain parallel and maintain a constant distance whiletranslating relative to each other. A “shear acoustic mode” is anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 400 are represented bythe curves 460, with the adjacent small arrows providing a schematicindication of the direction and magnitude of atomic motion. The degreeof atomic motion, as well as the thickness of the piezoelectric plate410, have been greatly exaggerated for ease of visualization. While theatomic motions are predominantly lateral (i.e. horizontal as shown inFIG. 4), the direction of acoustic energy flow of the excited primaryshear acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 5 is a schematic circuit diagram and layout for a high frequencyband-pass filter 500 using XBARs. The filter 500 has a conventionalladder filter architecture including three series resonators 510A, 510B,510C and two shunt resonators 520A, 520B. The three series resonators510A, 510B, and 510C are connected in series between a first port and asecond port (hence the term “series resonator”). In FIG. 5, the firstand second ports are labeled “In” and “Out”, respectively. However, thefilter 500 is bidirectional and either port may serve as the input oroutput of the filter. The two shunt resonators 520A, 520B are connectedfrom nodes between the series resonators to ground. A filter may containadditional reactive components, such as inductors, not shown in FIG. 5.All the shunt resonators and series resonators are XBARs. The inclusionof three series and two shunt resonators is exemplary. A filter may havemore or fewer than five total resonators, more or fewer than threeseries resonators, and more or fewer than two shunt resonators.Typically, all of the series resonators are connected in series betweenan input and an output of the filter. All of the shunt resonators aretypically connected between ground and the input, the output, or a nodebetween two series resonators.

In the exemplary filter 500, the three series resonators 510A, B, C andthe two shunt resonators 520A, B of the filter 500 are formed on asingle plate 530 of piezoelectric material bonded to a silicon substrate(not visible). Each resonator includes a respective IDT (not shown),with at least the fingers of the IDT disposed over a cavity in thesubstrate. In this and similar contexts, the term “respective” means“relating things each to each”, which is to say with a one-to-onecorrespondence. In FIG. 5, the cavities are illustrated schematically asthe dashed rectangles (such as the rectangle 535). In this example, eachIDT is disposed over a respective cavity. In other filters, the IDTs oftwo or more resonators may be disposed over a single cavity.

Each of the resonators 510A, 510B, 510C, 520A, 520B in the filter 500has resonance where the admittance of the resonator is very high and ananti-resonance where the admittance of the resonator is very low. Theresonance and anti-resonance occur at a resonance frequency and ananti-resonance frequency, respectively, which may be the same ordifferent for the various resonators in the filter 500. Inover-simplified terms, each resonator can be considered a short-circuitat its resonance frequency and an open circuit at its anti-resonancefrequency. The input-output transfer function will be near zero at theresonance frequencies of the shunt resonators and at the anti-resonancefrequencies of the series resonators. In a typical filter, the resonancefrequencies of the shunt resonators are positioned below the lower edgeof the filter's passband and the anti-resonance frequencies of theseries resonators are position above the upper edge of the passband.

FIG. 6 is a schematic cross-sectional view through a shunt resonator anda series resonator of a filter 600 that uses a dielectric frequencysetting layer to separate the resonance frequencies of shunt and seriesresonators. A piezoelectric plate 610 is attached to a substrate 620.Portions of the piezoelectric plate 610 form diaphragms spanningcavities 640 in the substrate 620. Interleaved IDT fingers, such asfinger 630, are formed on the diaphragms. A first dielectric layer 650,having a thickness t1, is formed over the IDT of the shunt resonator.The first dielectric layer 650 is considered a “frequency settinglayer”, which is a layer of dielectric material applied to a firstsubset of the resonators in a filter to offset the resonance frequenciesof the first subset of resonators with respect to the resonancefrequencies of resonators that do not receive the dielectric frequencysetting layer. The dielectric frequency setting layer is commonly SiO₂but may be silicon nitride, aluminum oxide, or some other dielectricmaterial. The dielectric frequency setting layer may be a laminate orcomposite of two or more dielectric materials.

A second dielectric layer 655, having a thickness t2, may be depositedover both the shunt and series resonator. The second dielectric layer655 serves to seal and passivate the surface of the filter 600. Thesecond dielectric layer may be the same material as the first dielectriclayer or a different material. The second dielectric layer may be alaminate or composite of two or more different dielectric materials.Further, as will be described subsequently, the thickness of the seconddielectric layer may be locally adjusted to fine-tune the frequency ofthe filter 600A. Thus, the second dielectric layer can be referred to asthe “passivation and tuning layer”.

The resonance frequency of an XBAR is roughly proportional to theinverse of the total thickness of the diaphragm including thepiezoelectric plate 610 and the dielectric layers 650, 655. Thediaphragm of the shunt resonator is thicker than the diaphragm of theseries resonator by the thickness t1 of the dielectric frequency settinglayer 650. Thus, the shunt resonator will have a lower resonancefrequency than the series resonator. The difference in resonancefrequency between series and shunt resonators is determined by thethickness t1.

FIG. 7A is a schematic plan view and FIG. 7B is a cross-sectional viewof an XBAR 700 with diaphragm support pedestals. Referring to FIG. 7A,the XBAR 700 includes an IDT 730 formed on a front surface (712 in FIG.7B) of a piezoelectric plate 710. The dashed line 745 is the perimeterof a cavity (740 in FIG. 7B) formed in a substrate (720 in FIG. 7B)behind the piezoelectric plate. A portion of the piezoelectric plate 710over the cavity 745 (i.e. within the dashed rectangle) forms a diaphragm(715 in FIG. 7B) suspended over the cavity. The IDT 730 includes a firstbus bar 732, a second bus bar 734, and a plurality of interleavedfingers, such as finger 736, extending alternately from the first andsecond bus bars 732, 734. The interleaved fingers are disposed on thediaphragm. Shaded areas indicate where the piezoelectric plate issupported by the substrate (720 in FIG. 7B) or by diaphragm supportpedestals extending from the substrate. “Pedestal” has its normalmeaning of “a supporting part”. A “diaphragm support pedestal” is a partthat extends between a substrate and a diaphragm to provide support tothe diaphragm.

FIG. 7B is a cross-sectional view of the XBAR 700 at a section plane E-Edefined in FIG. 7A. The piezoelectric plate 710 has a front surface 712and a back surface 714. The back surface 714 is attached to thesubstrate 720. A portion of the piezoelectric plate 710 forms adiaphragm 715 spanning the cavity 740 in the substrate 720. A conductorpattern including an IDT (730 in FIG. 7A) is formed on the front surface712 of the piezoelectric plate. Interleaved fingers of the IDT, such asfinger 736, are disposed on the diaphragm 715.

A plurality of diaphragm support pedestals, such as diaphragm supportpedestal 725, connect the diaphragm 715 to the substrate 720 within thecavity 740. Each support pedestal is aligned with a finger of the IDT730, which is to say each diaphragm support pedestal contacts the backside 714 of the piezoelectric plate in an area immediately opposite arespective IDT finger. When an RF signal is applied to the IDT 730, anelectric field is formed between the IDT fingers. The magnitude of theelectric field, and thus the atomic motion in the piezoelectric plate710, is relatively low beneath each IDT finger. Aligning the diaphragmsupport pedestals with IDT fingers may minimize the acoustic energycoupled through the diaphragm support pedestals to the substrate 720.

In the example of FIG. 7A and FIG. 7B, the diaphragm support pedestalsare ribs extending across the aperture AP of the XBAR along the lengthof all of the IDT fingers.

FIG. 8A is a detailed cross-sectional view of a diaphragm supportpedestal 825 that provides support to a piezoelectric plate 710. Thediaphragm support pedestal 825 is aligned with an IDT finger 836. Awidth wp of the diaphragm support pedestal 825 is less than or equal toa width w of the IDT finger 836.

The diaphragm support pedestal 825 includes a core 822 that extends froma substrate 720. The core 822 may be the same material as the substrate720. The core 822 may be a portion of substrate 720 that remained afterthe cavity 740 was etched into the substrate 720. The core 822 may be adifferent material from the substrate 720.

The diaphragm support pedestal 825 also includes a bonding layer 824that covers the core 822 and the substrate 720. The bonding layer is amaterial capable of bonding with the piezoelectric plate 710 using awafer bonding process. When the substrate 720 is silicon, the bondinglayer 824 may be silicon dioxide, aluminum oxide, another metal oxide,or some other material capable of bonding with the piezoelectric plate710.

FIG. 8B is a detailed cross-sectional view of another diaphragm supportpedestal 835 that provides support to a piezoelectric plate 710. Thediaphragm support pedestal 835 is aligned with an IDT finger 836. Awidth wp of the diaphragm support pedestal 835 is less than or equal toa width w of the IDT finger 836.

The diaphragm support pedestal 835 includes a base 832 that extends froma substrate 720. The base 832 may be the same material as the substrate720. The base 832 may be a portion of substrate 720 that remained afterthe cavity 740 was etched into the substrate 720. The base 832 may be adifferent material from the substrate 720.

The diaphragm support pedestal 835 also includes a bonding layer 834that covers at least the top of core 832 between the core 832 and thepiezoelectric plate 710. The bonding layer 834 is a material capable ofbonding with the piezoelectric plate 710 using a wafer bonding process.When the substrate 720 is silicon, the bonding layer 834 may be silicondioxide, aluminum oxide, another metal oxide, or some other materialcapable of bonding with the piezoelectric plate 710.

FIG. 9A is a schematic plan view and FIG. 9B is a cross-sectional viewof an XBAR 900 with diaphragm support pedestals. Referring to FIG. 9A,the XBAR 900 includes an IDT 930 formed on a front surface (912 in FIG.9B) of a piezoelectric plate 910. The dashed line 945 is the perimeterof a cavity (940 in FIG. 9B) formed in a substrate (920 in FIG. 9B)behind the piezoelectric plate. A portion of the piezoelectric plate 910within the dashed rectangle 945 forms a diaphragm suspended over thecavity. The IDT 930 includes a first bus bar 932, a second bus bar 934,and a plurality of interleaved fingers, such as finger 936, extendingalternately from the first and second bus bars 932, 934. The interleavedfingers are disposed on the diaphragm. Shaded areas indicate where thepiezoelectric plate is supported by the substrate (920 in FIG. 9B) or bydiaphragm support pedestals extending from the substrate.

FIG. 9B is a cross-sectional view of the XBAR 900 at a section plane F-Fdefined in FIG. 9A. The piezoelectric plate 910 has a front surface 912and a back surface 914. The back surface 914 is attached to thesubstrate 920. A portion of the piezoelectric plate 910 forms thediaphragm 915 spanning the cavity 940 in the substrate 920. A conductorpattern including the IDT 930 is formed on the front surface 912 of thepiezoelectric plate. Interleaved fingers of the IDT, such as finger 936,are disposed on the diaphragm 915.

A plurality of diaphragm support pedestals, such as diaphragm supportpedestal 925, connect the diaphragm 915 to the substrate 920 within thecavity 940. Each support pedestal is aligned with a finger of the IDT930, which is to say the diaphragm support pedestal contacts the backside 914 of the piezoelectric plate in an area immediately opposite arespective IDT finger. In the example of FIG. 9A and FIG. 9B, thediaphragm support pedestals are ribs that extend across the cavity 940under every third IDT finger. Placing a support pedestal under everythird IDT finger is exemplary. A diaphragm support pedestal may bealigned with every n'th IDT finger, where n is an integer between 2 and20.

FIG. 10 is a schematic plan view of an XBAR 1000 with diaphragm supportpedestals. The XBAR 1000 includes an IDT 1030 formed on a surface of apiezoelectric plate 1010. The dashed line 1045 is the perimeter of acavity formed in a substrate behind the piezoelectric plate. A portionof the piezoelectric plate 1010 within the dashed rectangle 1045 forms adiaphragm suspended over the cavity. The IDT 1030 includes a first busbar 1032, a second bus bar 1034, and a plurality of interleaved fingers,such as finger 1036, extending alternately from the first and second busbars 1032, 1034. The interleaved fingers are disposed on the diaphragm.Shaded areas indicate where the piezoelectric plate is supported by thesubstrate or by diaphragm support pedestals extending from thesubstrate.

The XBAR 1000 is divided into five sections 1050, 1060, 1070, 1080, 1090for the purpose of illustrating possible diaphragm support pedestalarrangements. The diaphragm support pedestals in section 1050 are ribs1055 that extend along roughly the center half of each IDT finger. Thediaphragm support pedestals in section 1060 are posts 1065 located atabout the center of each IDT finger. The diaphragm support pedestals insection 1070 are posts 1075 located at the ends of the IDT fingers. Thediaphragm support pedestals in section 1080 are posts 1085 located inalternating positions along the IDT fingers. The diaphragm supportpedestals in section 1090 include two posts 1095 aligned with each IDTfinger.

The diaphragm support pedestal arrangements in sections 1050, 1060,1070, 1080, and 1090 of the XBAR 1000 are examples of the nearlyunlimited number of arrangements of diaphragm support pedestals that arepossible. A diaphragm support pedestal may be a rib that supports adiaphragm along the entire aperture of an XBAR or may be a post with anapproximately square cross-section. A diaphragm support pedestal may beany size between these two extremes. None, one, two, or more diaphragmsupport pedestals may be aligned with each IDT finger. In all cases, thewidth of a diaphragm support pedestal will be less than or equal to thewidth of the IDT finger with which it is aligned. Any diaphragm supportpedestal shape or arrangement, such as those shown in FIG. 10, could bealigned with every n'th IDT finger, where n is an integer between 2 andabout 20, as previously shown in FIG. 9A and FIG. 9B for the case wheren=3.

Description of Methods

FIG. 11 is a simplified flow chart showing a process 1100 for making anXBAR or a filter incorporating XBARs. The process 1100 starts at 1105with a substrate and a plate of piezoelectric material and ends at 1195with a completed XBAR or filter. The flow chart of FIG. 11 includes onlymajor process steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 11.

The piezoelectric plate may be, for example, Z-cut, rotated ZY-cut orrotated YX cut lithium niobate. The substrate may preferably be silicon.The substrate may be some other material that allows formation of deepcavities by etching or other processing.

In a first embodiment of the process 1100, one or more cavities areformed in the substrate at 1110A. For example, a separate cavity may beformed for each resonator in a filter device. In some filters,resonators may be divided into sub-resonators connected in parallel. Inthis case, a separate cavity may be formed for each sub-resonator. Eachcavity may contain none, one, few, or many diaphragm support pedestals.Each diaphragm support pedestal is a portion of the substrate notremoved when the cavities are formed. The cavities and diaphragm supportpedestals may be formed using conventional photolithographic andanisotropic etching techniques. For example, when the substrate issilicon, the cavities and diaphragm support pedestals may be formedusing anisotropic reactive ion etching.

At 1120A, a bonding layer is deposited over the substrate including thecavities and the diaphragm support pedestals. The bonding layer is amaterial capable of bonding with the piezoelectric plate using a waferbonding process. When the substrate is silicon, the bonding layer may besilicon dioxide, aluminum oxide, another metal oxide, or some othermaterial capable of bonding with the piezoelectric plate. After thebonding layer is deposited, each support pedestal will be similar to thesupport pedestal 825 in FIG. 8A.

In a second embodiment of the process 1100, a bonding layer is depositedover a surface of the substrate at 1120B. When the substrate is silicon,the bonding layer may be silicon dioxide, aluminum oxide, another metaloxide, or some other material capable of bonding with the piezoelectricplate.

At 1110B, one or more cavities are formed in the substrate by etchingthrough the bonding layer deposited at 1120B into the substrate. Forexample, a separate cavity may be formed for each resonator in a filterdevice. In some filters, resonators may be divided into sub-resonatorsconnected in parallel. In this case, a separate cavity may be formed foreach sub-resonator. Each cavity may contain none, one, few, or manydiaphragm support pedestals. Each diaphragm support pedestal is aportion of the substrate not removed when the cavities are formed. Thecavities and diaphragm support pedestals may be formed usingconventional photolithographic and anisotropic etching techniques. Forexample, when the substrate is silicon, the cavities and diaphragmsupport pedestals may be formed using anisotropic reactive ion etching.

In either the first or second embodiments of the process 1100, thebonding layer is formed on all surfaces of the substrate and diaphragmsupport pedestals that will be bonded to the piezoelectric plate in asubsequent action.

At 1130, the piezoelectric plate is bonded to the substrate surroundingthe cavities and to the tops of the diaphragm support pedestals withinthe cavities. The piezoelectric plate and the substrate may be bonded bya wafer bonding process. Typically, the mating surfaces of the substrateand the piezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers. At the conclusion of the bonding, the bonding layer issandwiched between the piezoelectric plate and the substrate and betweenthe piezoelectric plate and the diaphragm support pedestals.

A conductor pattern, including IDTs of each XBAR, is formed at 1140 bydepositing and patterning one or more conductor layer on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 1140 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1140 using alift-off process. Photoresist may be deposited over the piezoelectricplate and patterned to define the conductor pattern. The conductor layerand, optionally, one or more other layers may be deposited in sequenceover the surface of the piezoelectric plate. The photoresist may then beremoved, which removes the excess material, leaving the conductorpattern.

At 1150, a front-side dielectric layer may be formed by depositing oneor more layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

The filter device is completed at 1160. Actions that may occur at 1160include depositing an encapsulation/passivation layer such as SiO₂ orSi₃O₄ over all or a portion of the device; forming bonding pads orsolder bumps or other means for making connection between the device andexternal circuitry; excising individual devices from a wafer containingmultiple devices; other packaging steps; and testing. Another actionthat may occur at 1160 is to tune the resonant frequencies of theresonators within the device by adding or removing metal or dielectricmaterial from the front side of the device. After the filter device iscompleted, the process ends at 1195.

FIG. 12 is a simplified flow chart showing another process 1200 formaking an XBAR or a filter incorporating XBARs. The process 1200 startsat 1205 with a substrate and a plate of piezoelectric material and endsat 1295 with a completed XBAR or filter. The flow chart of FIG. 12includes only major process steps. Various conventional process steps(e.g. surface preparation, cleaning, inspection, baking, annealing,monitoring, testing, etc.) may be performed before, between, after, andduring the steps shown in FIG. 12.

The piezoelectric plate may be, for example, Z-cut, rotated ZY-cut orrotated YX cut lithium niobate. The substrate may preferably be silicon.The substrate may be some other material that allows formation of deepcavities by etching or other processing.

One or more cavities are formed in the substrate at 1210. For example, aseparate cavity may be formed for each resonator in a filter device. Insome filters, resonators may be divided into sub-resonators connected inparallel. In this case, a separate cavity may be formed for eachsub-resonator. Each cavity may contain none, one, few, or many diaphragmsupport pedestals. Each diaphragm support pedestal is a portion of thesubstrate not removed when the cavities are formed. The cavities anddiaphragm support pedestals may be formed using conventionalphotolithographic and anisotropic etching techniques. For example, whenthe substrate is silicon, the cavities and diaphragm support pedestalsmay be formed using anisotropic reactive ion etching.

At 1215, the cavities formed at 1210 are filled with a sacrificialmaterial that will be subsequently removed. The sacrificial material maybe different from the material of the substrate. For example, when thesubstrate is silicon, the sacrificial material may be an oxide, anitride, a glass, or a polymer material. The sacrificial material may bedeposited on the substrate with sufficient thickness to fill thecavities. The excess material may then be removed, for example bychemo-mechanical polishing, to leave a flat surface suitable for bondingto the piezoelectric plate. The excess material may be removedsufficiently to expose the tops of the diaphragm support pedestals.

At 1220, a bonding layer is deposited over the substrate including thefilled cavities and the diaphragm support pedestals. The bonding layeris a material capable of bonding with the piezoelectric plate using awafer bonding process. When the substrate is silicon, the bonding layermay be silicon dioxide, aluminum oxide, another metal oxide, or someother material capable of bonding with the piezoelectric plate.

At 1230, the piezoelectric plate is bonded to the substrate surroundingthe cavities, the fill material in the cavities and the tops of thediaphragm support pedestals within the cavities. The piezoelectric plateand the substrate may be bonded by a wafer bonding process. Typically,the mating surfaces of the substrate and the piezoelectric plate arehighly polished. One or more layers of intermediate materials, such asan oxide or metal, may be formed or deposited on the mating surface ofone or both of the piezoelectric plate and the substrate. One or bothmating surfaces may be activated using, for example, a plasma process.The mating surfaces may then be pressed together with considerable forceto establish molecular bonds between the piezoelectric plate and thesubstrate or intermediate material layers. At the conclusion of thebonding, the bonding layer is sandwiched between the piezoelectric plateand the substrate and between the piezoelectric plate and the diaphragmsupport pedestals.

A conductor pattern, including IDTs of each XBAR, is formed at 1240 aspreviously described. At 1250, a front-side dielectric layer or layersmay be formed as previously described.

At 1255, the sacrificial material is removed from the cavities using anetchant or solvent introduced through openings in the piezoelectricplate. After the sacrificial material is removed, portions of thepiezoelectric plate form diaphragms suspended over the cavities andpartially support by the diaphragm support pedestals.

The filter device is completed at 1260. Actions that may occur at 1260include depositing an encapsulation/passivation layer such as SiO₂ orSi₃O₄ over all or a portion of the device; forming bonding pads orsolder bumps or other means for making connection between the device andexternal circuitry; excising individual devices from a wafer containingmultiple devices; other packaging steps; and testing. Another actionthat may occur at 1260 is to tune the resonant frequencies of theresonators within the device by adding or removing metal or dielectricmaterial from the front side of the device. After the filter device iscompleted, the process ends at 1295.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substratehaving a surface; a piezoelectric plate having front and back surfaces,the back surface attached to the surface of the substrate except for aportion of the piezoelectric plate forming a diaphragm that spans acavity in the substrate; an interdigital transducer (IDT) formed on thefront surface of the piezoelectric plate such that interleaved fingersof the IDT are disposed on the diaphragm, and one or more diaphragmsupport pedestals extending from a bottom surface of the cavity to thediaphragm.
 2. The device of claim 1, wherein the one or more diaphragmsupport pedestals is a portion of substrate that remained after thecavity was etched into the substrate.
 3. The device of claim 1, whereinthe one or more diaphragm support pedestals is a different material fromthe substrate.
 4. The device of claim 1, wherein: when an RF signal isapplied to the IDT, an electric field is formed between the IDT fingers;the magnitude of the electric field, and thus the atomic motion in thepiezoelectric plate, is relatively low beneath each IDT finger; and eachdiaphragm support pedestal contacts the back side of the piezoelectricplate in an area immediately opposite a respective IDT finger tominimize the acoustic energy coupled through the diaphragm supportpedestals to the substrate.
 5. The device of claim 1, wherein each ofthe one or more diaphragm support pedestals is aligned with a respectivefinger of the interleaved fingers of the IDT, wherein the one or morediaphragm support pedestals comprise diaphragm support pedestals alignedwith every n fingers of the IDT, and where n is an integer greater thanor equal to two and less than or equal to twenty.
 6. The device of claim1, wherein a width of each of the one or more diaphragm supportpedestals is less than or equal to a width of each finger of theinterleaved fingers of the IDT.
 7. The device of claim 6, wherein alength of each of the one or more diaphragm support pedestals is greaterthan or equal to the width of the diaphragm support pedestals and lessthan or equal to an aperture of the IDT.
 8. The device of claim 1,further comprising a bonding layer sandwiched between the piezoelectricplate and the substrate and between the piezoelectric plate and the oneor more diaphragm support pedestals.
 9. A method of fabricating anacoustic resonator device, comprising: forming a cavity in a substrate,one or more diaphragm support pedestals disposed within the cavity;depositing a bonding layer over the substrate, the cavity, and the oneor more diaphragm support pedestals; bonding a piezoelectric plate tothe bonding layer such that a portion of the piezoelectric plate forms adiaphragm spanning the cavity with each of the one or more diaphragmsupport pedestals extending from a bottom surface of the cavity to thebonding layer; and forming an interdigital transducer (IDT) on a frontsurface of the piezoelectric plate such that interleaved fingers of theIDT are disposed on the diaphragm.
 10. The method of claim 9, whereinthe piezoelectric plate and the IDT are configured such that a radiofrequency signal applied to the IDT excites a primary shear acousticmode in the diaphragm.
 11. The method of claim 9, wherein each of theone or more diaphragm support pedestals is aligned with a respectivefinger of the interleaved fingers of the IDT.
 12. The method of claim11, wherein a width of each of the one or more diaphragm supportpedestals is less than or equal to a width of each finger of theinterleaved fingers of the IDT.
 13. A method of fabricating an acousticresonator device, comprising: depositing a bonding layer over asubstrate; forming a cavity in the substrate and the bonding layer, oneor more diaphragm support pedestals disposed within the cavity; bondinga piezoelectric plate to the bonding layer and the one or more diaphragmsupport pedestals such that a portion of the piezoelectric plate forms adiaphragm spanning the cavity with each of the one or more diaphragmsupport pedestals extending from a bottom surface of the cavity to thediaphragm; and forming an interdigital transducer (IDT) on the frontsurface of the piezoelectric plate such that interleaved fingers of theIDT are disposed on the diaphragm.
 14. The method of claim 13, whereinthe piezoelectric plate and the IDT are configured such that a radiofrequency signal applied to the IDT excites a primary shear acousticmode in the diaphragm.
 15. The method of claim 13, wherein each of theone or more diaphragm support pedestals is aligned with a respectivefinger of the interleaved fingers of the IDT.
 16. The method of claim15, wherein a width of each of the one or more diaphragm supportpedestals is less than or equal to a width of each finger of theinterleaved fingers of the IDT.
 17. A method of fabricating an acousticresonator device, comprising: forming a cavity in a substrate, one ormore diaphragm support pedestals disposed within the cavity; filling thecavity with a sacrificial material; depositing a bonding layer over thesubstrate, the diaphragm support pedestals and the sacrificial material;bonding a piezoelectric plate to the bonding layer and the one or morediaphragm support pedestals such that each of the one or more diaphragmsupport pedestals extend from a bottom surface of the cavity to thebonding layer; forming an interdigital transducer (IDT) on the frontsurface of the piezoelectric plate such that interleaved fingers of theIDT are disposed on the diaphragm; and removing the sacrificial materialsuch that a portion of the piezoelectric plate forms a diaphragmspanning the cavity.
 18. The method of claim 17, wherein thepiezoelectric plate and the IDT are configured such that a radiofrequency signal applied to the IDT excites a primary shear acousticmode in the diaphragm.
 19. The method of claim 17, wherein each of theone or more diaphragm support pedestals is aligned with a respectivefinger of the interleaved fingers of the IDT.
 20. The method of claim19, wherein a width of each of the one or more diaphragm supportpedestals is less than or equal to a width of each finger of theinterleaved fingers of the IDT.